I am pleased to have guest blogger James Wilson, Director of Marketing, Timing Products, Silicon Labs with the following informative blog critical to next-gen communications systems.
Mobile traffic is increasing at an exponential rate due to the popularity of data-intensive services such as video streaming and mobile internet. To expand coverage and increase capacity, network operators are increasingly deploying small cells and pico cells as a solution of choice. These small form factor base stations can be deployed between larger macro base stations to enable network densification in congested, high-traffic urban environments. Cost is critically important given the sheer number of small cells and pico cells that need to be deployed. It is important to note that equipment cost is not the only concern in these applications. Low-cost, low-power base stations also require cost-effective network synchronization.
In legacy mobile backhaul applications, circuit-switched TDM networks such as SONET/SDH and PDH were used to natively carry a timing reference over the physical layer. While reliable, these solutions proved to be too costly and not scalable because they relied heavily on leased line T1/E1 infrastructure. As the mobile backhaul industry has matured and transitioned from legacy circuit-switched networks to high bandwidth packet-based networks, it too has required more cost-effective network synchronization.
A similar story is unfolding now in broadband access applications including Passive Optical Networking (PON) and xDSL applications. Customers are interested in implementing lower cost network synchronization solutions while transitioning to higher bandwidth packet network uplinks.
The industry responded by developing multiple synchronization solutions for packet networks. There are three main methods for delivering synchronization over packet networks. The first is Synchronous Ethernet (SyncE), which provides frequency synchronization using physical-layer-based timing. SyncE is defined by the Internet Telecommunication Union Telecommunication Standards Sector (ITU-T) G.8261, G.8262 and G.8264. The second approach involves packet-based timing using IEEE 1588-2008 Precision Time Protocol (PTP)), as defined by ITU-T G.8265. IEEE 1588-2008 PTP provides frequency, time and phase synchronization. Finally, global navigation satellite systems (GNSS) can be used to provide time and phase synchronization.
The specific type of synchronization method employed in a particular system is highly application-specific. Some applications use SyncE-only to provide frequency synchronization. Other applications including LTE-TDD require tight phase/time alignment to ensure quality of service and tight call coordination between different base stations. As a result, LTE-TDD deployments typically use a mixture of GNSS and IEEE 1588 to provide timing synchronization.
Another important variable in enabling cost-effective packet network synchronization is the network synchronizer clock IC. These devices provide both physical layer as well as packet layer synchronization. Conventional network synchronizer clocks ICs that support synchronization over packet networks rely on chip architectures that borrow heavily from legacy Stratum 3/3E clock IC implementations used in wired networks, which are not optimized for size, power or performance. This trend toward complexity and over-integration runs counter to the communication industry’s need for simple, low-cost, low-power network synchronization. It also runs counter to the natural trend in the industry to incorporate an increasing number of clock management functions in FPGA to minimize the overall solution cost.
While conventional synchronization solutions tend to rely on expensive, rigid architectures, recent advancements in digital phase-locked-loop (PLL) technology are now available that greatly minimize the cost and complexity of frequency, time and phase synchronization over packet networks. These solutions simplify the design of space-constrained, low-power wireless base stations and networking equipment.